Object-Oriented Programming concepts recap with Java examples

Concept

• Software development paradigm
• We should represent our system concepts using classes
• Classes deals with common behaviour to all of its different instances (objects):

• Objects have their own memory
• Object communicate between them sending and receiving messages

Visibility and inheritance

public, protected, and private visibility keywords

• When do you use each one?
  • Guilt presumption
  • Simplify our classes API (exposed methods) => Easier to understand, easier to be SRP compliant, avoid having to maintain public methods because others are coupled to them
• Question:
  • Which would be the output of the Child#visibilityTest method?
  • Solution: ChildShould. Possible answers:

        "Child#privateMethod Child#protectedMethod Child#publicMethod" // a
        "Parent#privateMethod Child#protectedMethod Child#publicMethod" // b
        "Parent#privateMethod Parent#protectedMethod Child#publicMethod" // c
        "Parent#privateMethod Child#protectedMethod Parent#publicMethod" // d
        "Parent#privateMethod Parent#protectedMethod Parent#publicMethod" // e
        // It doesn't compile // f

static keyword

• What is it for?
• Question:
  • Which would be the output for the following getTotal calls?
  • Solution: CounterShould. Possible answers:

Counter counterA = new Counter();
Counter counterB = new Counter();
Counter counterC = new Counter();

counterA.increaseTotal();
counterA.increaseTotal();
counterA.increaseTotal();

counterB.increaseTotal();
counterB.increaseTotal();

counterC.increaseTotal();

// a:
counterA.getTotal(); // 0
counterB.getTotal(); // 0
counterC.getTotal(): // 0

// b:
counterA.getTotal(); // 6
counterB.getTotal(); // 6
counterC.getTotal(): // 6

// c:
counterA.getTotal(); // 3
counterB.getTotal(); // 5
counterC.getTotal(): // 6

// c:
counterA.getTotal(); // 6
counterB.getTotal(); // 3
counterC.getTotal(): // 1

final keyword

• What does it do in attributes?
  • Does not allow to redefine them
• What does it do in methods?
  • Does not allow to override them
• What does it do in classes?
  • Does not allow to inherit from them
• When we should use it?
  • Same reasoning as with the visibility keywords: Guilt presumption.
  • Why: Make the next developer think twice before extending from it.
  • Key concept: Composition over Inheritance.

abstract classes vs interfaces

• What's the difference?
  • Interfaces:
    • Doesn't allow to implement method bodies. It only allow us to declare method contracts/headers. <- True until Java8
    • They're great because as they have fewer capabilities, they are easier to read and understand without letting us mess up adding behaviour.
    • A class can implement different interfaces.
  • Abstract classes:
    • Allow to implement method bodies.
    • A class can only extend from one abstract class.
• When we should use abstract classes?
  • Opinion: Almost never. Just exceptional cases. We should have a very big reason to do so 🙂
• When we should use interfaces?
  • Opinion: In order to decouple from infrastructure* stuff.
  • *Infrastructure: behaviour related to a third party library or component (Postgres DB, AWS SDK, Slack SDK, MailChimp API…)
  • Usage example:

interface ProductRecommender
{
    Recommendations findFor(ProductId productId);
}

final class BlueknowProductRecommender implements ProductRecommender
{
    @Override
    public Recommendations findFor(ProductId productId) {
        // Call to the Blueknow service API
        // Parse the JSON response into a `Recommendations` class instance
        return recommendations;
    }
}

Example:

• Context:
  • We have a Builder system in order to build our applications
  • We want to notify the development team once the build is ready to be deployed
  • We're testing different messaging apps such as Slack and HipChat
  • In order to do not miss our notifications, we want to be notified through the #dev-notifications Slack channel, and through the [email protected] mailing list
• Questions:
  • How would you model these different classes and their interactions?